1. Statement of the Technical Field
The inventive arrangements relate generally to the field of communications, and more particularly to phased array antennas.
2. Description of the Related Art
Existing microwave antennas include a wide variety of configurations for various applications, such as satellite reception, remote broadcasting, or military communication. The desirable characteristics of low cost, light-weight, low profile and mass producibility are provided in general by printed circuit antennas. The simplest forms of printed circuit antennas are microstrip antennas wherein flat conductive elements are spaced from a single essentially continuous ground element by a dielectric sheet of uniform thickness. An example of a microstrip antenna is disclosed in U.S. Pat. No. 3,995,277 to Olyphant.
The antennas are designed in an array and may be used for communication systems such as identification of friend/foe (IFF) systems, personal communication service (PCS) systems, satellite communication systems, and aerospace systems, which require such characteristics as low cost, light weight, low profile, and a low sidelobe.
The bandwidth and directivity capabilities of such antennas, however, can be limiting for certain applications. While the use of electromagnetically coupled microstrip patch pairs can increase bandwidth, obtaining this benefit presents significant design challenges, particularly where maintenance of a low profile and broad beam width is desirable. Also, the use of an array of microstrip patches can improve directivity by providing a predetermined scan angle. However, utilizing an array of microstrip patches presents a dilemma. The scan angle can be increased if the array elements are spaced closer together, but closer spacing can increase undesirable coupling between antenna elements thereby degrading performance.
Furthermore, while a microstrip patch antenna is advantageous in applications requiring a conformal configuration, e.g. in aerospace systems, mounting the antenna presents challenges with respect to the manner in which it is fed such that conformality and satisfactory radiation coverage and directivity are maintained and losses to surrounding surfaces are reduced. More specifically, increasing the bandwidth of a phased array antenna with a wide scan angle is conventionally achieved by dividing the frequency range into multiple bands.
One example of such an antenna is disclosed in U.S. Pat. No. 5,485,167 to Wong et al. This antenna includes several pairs of dipole pair arrays each tuned to a different frequency band and stacked relative to each other along the transmission/reception direction. The highest frequency array is in front of the next lowest frequency array and so forth.
This approach may result in a considerable increase in the size and weight of the antenna while creating a Radio Frequency (RF) interface problem. Another approach is to use gimbals to mechanically obtain the required scan angle. Yet, here again, this approach may increase the size and weight of the antenna and result in a slower response time.
Thus, there is a need for a lightweight phased array antenna with a wide frequency bandwidth and a wide scan angle, and that is conformally mountable to a surface. Such a need has been met through the use of current sheet arrays or dipole layers using interdigital capacitors that increase coupling by lengthening the capacitor xe2x80x9cdigitsxe2x80x9d or xe2x80x9cfingersxe2x80x9d that result in additional bandwidth as discussed in U.S. Pat. No. 6,417,813 to Durham (""813 Patent) and assigned to the assignee herein. Some antennas of this structure exhibit a significant gain dropout at particular frequencies in the desired operational bandwidth. Thus, a need exists for a lightweight phased array antenna with a wide frequency bandwidth and wide scan angle that is still conformally mountable to a surface and is further not subject to the gain dropout discussed above.
Moreover, there is also a need for feedthrough lens antennas as discussed in the ""813 Patent, that also overcomes the gain dropout problem. Feedthrough lens antennas may be used in a variety of applications where it is desired to replicate an electromagnetic (EM) environment present on the outside of a structure within the structure over a particular bandwidth. For example, a feedthrough lens may be used to replicate signals, such as cellular telephone signals, within a building or airplane which may otherwise be reflected thereby. Furthermore, a feedthrough lens antenna may be used to provide a highpass filter response characteristic, which may be particularly advantageous for applications where very wide bandwidth is desirable. An example of such a feedthrough lens antenna is disclosed in the patent to Wong et al. The feedthrough lens structure disclosed in the Wong et al patent includes several of the multiple layered phased array antennas discussed above. Yet, the above noted limitations will correspondingly be present when such antennas are used in feedthrough lens antennas.
In a first aspect of the present invention, a phased array antenna comprises a substrate and an array of dipole antenna elements thereon where each dipole antenna element comprises a medial feed portion and a pair of legs extending outwardly therefrom. Adjacent legs of adjacent dipole antenna elements preferably include respective spaced apart end portions. The phased array antenna further comprises at least one dielectric layer between the substrate and a ground plane and at least one conductive plane adjacent to the substrate for providing additional coupling between adjacent dipole antenna elements.
In a second aspect of the present invention, a phased array antenna comprises a current sheet array on a substrate, at least one dielectric layer between the current sheet array and a ground plane and at least one conductive plane adjacent to the substrate for providing additional coupling between adjacent dipole antenna elements of the current sheet array.
In a third aspect of the present invention, a method for making a phased array antenna comprises the steps of providing a substrate, forming an array of dipole antenna elements on the substrate to define the phased array antenna, each dipole antenna element comprising a medial feed portion and a pair of legs extending outwardly therefrom, and positioning and shaping respective spaced apart end portions of adjacent legs of adjacent dipole antenna elements to provide increased capacitive coupling between the adjacent dipole antenna elements, and providing a conductive plane adjacent to the array of dipole antenna elements to provide further capacitive coupling between the adjacent dipole antenna elements.
The spaced apart end portions have a predetermined shape and are relatively positioned to provide increased capacitive coupling between the adjacent dipole antenna elements. Preferably, the spaced apart end portions in adjacent legs comprise interdigitated portions, and each leg comprises an elongated body portion, an enlarged width end portion connected to an end of the elongated body portion, and a plurality of fingers, e.g. four, extending outwardly from said enlarged width end portion.
The wideband phased array antenna has a desired frequency range and the spacing between the end portions of adjacent legs is less than about one-half a wavelength of a highest desired frequency. Also, the array of dipole antenna elements may include first and second sets of orthogonal dipole antenna elements to provide dual polarization. A ground plane is preferably provided adjacent the array of dipole antenna elements and is spaced from the array of dipole antenna elements less than about one-half a wavelength of a highest desired frequency.
Preferably, each dipole antenna element comprises a printed conductive layer, and the array of dipole antenna elements are arranged at a density in a range of about 100 to 900 per square foot. The array of dipole antenna elements is sized and relatively positioned so that the wideband phased array antenna is operable over a frequency range of about 2 to 30 Ghz, and at a scan angle of about xc2x160 degrees. There may be at least one dielectric layer on the array of dipole antenna elements, and the flexible substrate may be supported on a rigid mounting member having a non-planar three-dimensional shape.
Features and advantages in accordance with the present invention are also provided by a method of making a wideband phased array antenna including forming an array of dipole antenna elements on a flexible substrate, where each dipole antenna element comprises a medial feed portion and a pair of legs extending outwardly therefrom. Forming the array of dipole antenna elements includes shaping and positioning respective spaced apart end portions of adjacent legs of adjacent dipole antenna elements to provide increased capacitive coupling between the adjacent dipole antenna elements. Shaping and positioning the respective spaced apart end portions preferably comprises forming interdigitated portions.